Ammeters?

Hello all I'm pretty new to the electric bike thing, but am quite excited to have my bike conversion up and going as of this weekend. I currently have a giant mountain bike with a transmagnetic 800W motor along with a Wilderness 50A BLDC controller. I fabricated a timing belt style drive and am running three 12 volt 12AH SLA batteries. This has been an interesting endeavor mainly because none of my equipment came with any sort of directions/wiring diagrams and secondly because the companies themselves arent quite in the know about the product they carry.

Anyhow enough of the history my question is regarding ammeters for use with the 36 volt system I have. I've noticed a problem with my motor sometimes cutting out upon start up so I decided to start looking for an ammeter to see what's going on. I've been looking at some different automotive style ammeters that are designed for 12 volts in fact I picked one up today to see if it might work at a higher voltage. It's an analog product so I decided to try it and see if it might be accurate. When juicing my motor I'm getting no more than 3-4amps. Now because of the motor and controller specs I expected a much larger number especially on motor start up. I'm familiar with current voltage and power measurements and how to figure each, but am not familiar with why or why not a meter like this would be inaccurate with a higher voltage. I understand that in the simplest form it's measuring the magnetic force produced by the current, but am wondering if anyone on here might have a little more insight as to why, why not or how you could make something like this to measure higher voltages accurately.

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Weird. Off the top of my head, I can't think of any reason why it wouldn't work at higher voltages. (Well, actually, I could, but they all end with the meter being way OVER the actual amperage.)

Perhaps because it's designed for crazy high amperage (on the order of hundreds) it's not accurate for the kind of current levels a bike uses?

Can you take it apart and show pictures? I've never owned an analogue meter, let alone one made just for car batteries.

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It may be how you wired it up (no offense!). The amp meter needs to be in series with the batteries or the motor to give either battery or motor amps. All of the current needs to pass through the amp meter shunt for it to be accurate.

I'm not sure how all amp meters work, but my guess is that it has a "shunt" which is really a resistor. When a current is passed through any resistor there will be a change in potential, and this can be measured. This change in potential is proportional to the current. The change in potential will be the same for a given current not matter what the voltage applied to the circuit is.

I'm not sure how all amp meters work, but my guess is that it has a "shunt" which is really a resistor. When a current is passed through any resistor there will be a change in potential, and this can be measured. This change in potential is proportional to the current. The change in potential will be the same for a given current not matter what the voltage applied to the circuit is.

On my Lectra I have an ammeter that works this way. I don't know much about how it works, just that the ammeter connects across this metal gizmo and measures current. That's better than wiring the main power circuit through to the handlebar so it can be displayed on a meter. I just yahoogled "ammeter shunt" and these came up, they appear to explain more.

The Paktrakr current measurer uses a hall effect detector rather than a shunt. There is a picture here: http://paktrakr.com/Shopping.html ... it consists of a rectangular thingymajob through which you run the main power cable. "Unlike a shunt resistor system, the PakTrakr current sensor is completely isolated from the pack, wastes no energy, and may be installed anywere in the pack."

I'm not sure how all amp meters work, but my guess is that it has a "shunt" which is really a resistor. When a current is passed through any resistor there will be a change in potential, and this can be measured. This change in potential is proportional to the current. The change in potential will be the same for a given current not matter what the voltage applied to the circuit is.

I must disagree with your statement. "The change in potential will be the same for a given current not matter what the voltage applied to the circuit is." First, potential and voltage are the same thing, so the statement makes no sense. The amount of voltage applied changes the current flow through a shunt. As you can see by the Ohms Law chart I = E / R so if R is 10 and E is 10 then I would be 1. 10/10=1 If we increase E (voltage) to 20 and do not change R (shunt) we get 20/10 so now I = 2. This is consistent and the amp meter should show this to be true. It may be that the meter scale is just not accurate enough to be used in this application.

Hey guys I really appreciate the help so far. I finally picked up a 9volt for my DMM today and began testing out the voltage to see what was going on. Though the batteries were not fully charged they sat at 37.4 volts. I then twisted my throttle and held the bike in place while watching the voltage and witnessed a normal drop of about a volt. It held steady at this for a second then the controller cut out.

I can only assume the BL Wilderness 50A controller doesnt like starting from a stop. This makes sense because it never happens after I'm going over 5 mph or so. Is it normal for the products to ship without any literature? Neither my motor or controller came with anything so it's sorta left me in the dark on what to expect. Anyhow if this is the case and the controller just doesnt like starting from a stop do you guys know of any way to get around this? The controller looks like it's a tank and is sealed inside with a half inch of clear rubbery material I'm assuming for water tightness.

Chas, nice chart! Sorry for my confusing statement. It's difficult for me to think of how to explain this...

What I meant was:

The potential change is proportional to the current, in the manner: E = R x I (from your chart, or ohms law). If the resistance is known for a portion of the circuit, than the current can be calculated by measuring E of the portion of the circuit that has a known resistance (or in other words the "shunt").

If 1,000v causes 1 amp to flow through the entire circuit, than the E measured across the shunt will be 1 * R.

If 1v causes 1 amp to flow through the entire circuit, than the E measured across the shunt will still be 1 * R.

So, E for the portion of the circuit that is the shunt will remain a function of the amount of current flowing, no matter what the voltage that is applied to the entire circuit to cause the given amount of current flow.

Please tell me how that doesn't make sense? (really, I'm not being sarcastic, just having difficult putting a concept into words).

I may be off base here, but since you said you did not know a lot of stuff from the suppliers, I'd point out here that there are controllers that are made to behave in this manner. Some places have laws against a motorized bike that does not require pedalling. Thus the controller is made to kick in when it senses the motion going past a certtain speed. Actually, this info ought to be available somewhere with the part number of your controller

It is still a little muddy but I do understand what you are trying to say and it is correct. I did not mean to offend but it was not clear in your first post so I was trying to clarify for myself and others here trying to learn.